Lesson 2 of 4
Investigate an energy change
Make the measurement large enough to interpret fairly.
O-Level 6092 (2026) / SEC G3 K324 (2027)
I can explain enthalpy-change sign.Syllabus
Syllabus K324, 9(a). Explain enthalpy-change sign
Investigate an energy change
Make the measurement large enough to interpret fairly.
Measure the initial temperatures, mix the specified quantities in an insulated cup, stir consistently and record the highest or lowest temperature reached. A lid reduces energy transfer to the air; the probe should measure the mixture rather than rest on the cup. Use the same volumes, concentrations and apparatus when comparing repeats.
Worked exampleInterpret a cooling experiment
Water starts at 24 degrees C. After a salt dissolves, the lowest temperature is 18 degrees C, before slowly returning towards 24.
Try it first. Then show the worked stepsHide the worked steps
- The initial fall is evidence that the dissolving process absorbed energy.
- The change in the measured temperature is 18 - 24 = -6 degrees C.
- The later warming occurs as the cooler solution gains energy from the room.
Dissolving is endothermic. The later return towards room temperature does not change that classification.
A smaller observed temperature change may result from heat exchange with the room, a different amount of reacting substance or a different amount of water being warmed. It need not mean a different reaction type. Report measurements and limitations rather than claiming that temperature change alone is an energy value in joules.
Check your understanding
How would heat loss to the room affect the measured temperature rise in an exothermic experiment?
Show the answer
The measured rise would be smaller than the true rise.
Some of the energy released escapes to the room instead of warming the solution, so the thermometer under-reads. The reaction itself is still exothermic.